The research may help explain why water quality tests don’t always accurately capture health risks for swimmers

“The take-home lesson is that E. coli that produce Shiga toxin persisted longer in recreational water than E. coli that don’t produce this toxin.”

Gerald Koudelka, Professor of biological sciences

University at Buffalo

The protist Tetrahymena hunts E. coli in this photo
illustration, which features a microscope image of Tetrahymena
(left). A new study finds that Shiga toxin may help E. coli
survive in the face of such predation.

BUFFALO, N.Y. — A toxin dangerous to humans may help E.
coli fend off aquatic predators, enabling strains of E.
coli that produce the toxin to survive longer in lake water
than benign counterparts, a new study finds.

“The take-home lesson is that E. coli that produce
Shiga toxin persisted longer in recreational water than E.
coli that don’t produce this toxin,” said UB
Professor of Biological Sciences Gerald Koudelka, PhD, who led the
study. “This is because the toxin appears to help E.
coli resist predation by bacterial grazers.”

The findings have implications for water quality testing. They
suggest that measuring the overall population of E. coli in
a river or lake — as many current tests do — may be a
poor way to find out whether the water poses a danger to
swimmers.

Past research has shown that overall E. coli
concentrations don’t always correlate with the levels of
dangerous, Shiga toxin-producing E. coli present in the
water, Koudelka said. His new study provides one possible
explanation for why this might be.

E. coli, short for Escherichia coli, is a bacteria
found in human and animal intestines. Most types of E. coli
are harmless. But those that produce Shiga toxin can make people
very sick, causing symptoms such as hemorrhagic diarrhea. Severe
cases can lead to death.

In their new study, Koudelka and his colleagues obtained water
samples from Presque Isle State Park and Mill Creek Stream, both in
northern Pennsylvania. The water contained protists — tiny,
single-cellular creatures that feed on E. coli.

To test how Shiga toxin affects E. coli’s survival,
the scientists placed several different strains of E. coli
into the water samples: three strains of Shiga toxin-encoding E.
coli (STEC), and three strains of E. coli that did not
produce the toxin.

The results: The toxin producers fared much better against the
grazing protists than their toxin-free counterparts. Over 24 hours,
STEC populations fell by an average of 1.4-fold, in contrast to
2.5-fold for the Shiga-free bacteria.

The STEC strain that produced the most Shiga toxin also lasted
the longest, persisting in water for about 48 hours before
declining in numbers.

Each E. coli strain was tested in its own experiment (as
opposed to one big experiment that included all six). All of the
STEC strains studied were ones that had previously caused illness
in humans.

The findings add to evidence suggesting that current water
quality tests may not capture the whole story when it comes to
E. coli danger in recreational waters, Koudelka said.

“If you’re only testing generally for fecal
indicator bacteria, you could miss the danger because it’s
possible to have low levels of E. coli overall, but have
most of that E. coli be of the STEC variety,” he said.
“This would be worse than having a large E. coli
population but no STEC.”

The opposite problem can also occur, Koudelka said.

“You could have high E. coli populations in a lake,
but absolutely no STEC,” he said. “This is the economic
part of it: It’s a problem because you might have a beach
that’s closed for days even though it’s
safe.”

The study was funded by the National Science Foundation and
Mercyhurst University, and the research is part of Koudelka’s
ongoing investigations into the lives of aquatic microbes,
including bacteria and protists. He is particularly interested in
how one species may help regulate the level of others through
predation and other interactions. Koudelka’s work has shown
that Shiga and other dangerous toxins probably arose as
antipredator defenses in what he calls a “microbiological
arms race,” and not to kill humans.

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